Solar cells manufacturers routinely perform inspection on their solar wafers. This is to ensure that any defective solar wafers are identified so as to control the quality of the solar cells.
A solar wafer is a thin slice of silicon crystal that is commonly used in the fabrication of solar cells. A solar wafer serves as a substrate for solar cells and undergoes a series of fabrication processes, for example deposition, etching and patterning, before becoming a functional solar cell. It is therefore very critical to maintain the quality of solar wafers from the beginning of the fabrication processes in order to improve production yield and reduce production cost.
Micro-crack is a common defect found in solar wafers, which is extremely difficult to detect because some micro-cracks are invisible to the human eyes and even to optical microscopes. One method of detecting micro-cracks in solar wafers involves the use of infrared imaging technique. Solar wafers are made from silicon of high purity and appear opaque under visible light. However, due to silicon's band-gap energy level, solar wafers appear transparent when illuminated with light having a wavelength larger than 1127 nm.
Light having a wavelength of 1127 nm is classified as near infrared (NIR) radiation. NIR is invisible to the human eye but is detectable by most commercial CCD or CMOS infrared cameras. Examples of infrared light sources are Light Emitting Diodes (LED), tungsten lamp and halogen lamp.
As infrared light is capable of penetrating through a solar wafer made from silicon, it is possible to examine the internal structure of the solar wafer by displacing the solar wafer in between the infrared camera and light source.
Solar wafers are manufactured in a production line at high volume, typically at a rate of one wafer per second. A solar wafer typically has a rectilinear shape and a surface dimension of between 100 mm by 100 mm and 210 mm by 210 mm. The solar wafer also has a typical thickness of between 150 μm to 250 μm. A conventional high speed imaging system is used for inspecting the solar wafers. Most conventional high speed imaging system uses a line-scan CCD/CMOS camera that has a resolution of up to 12000 (12K) pixels.
FIG. 1a shows a conventional high speed imaging system 10. The conventional high speed imaging system 10 consists of a computer 12 and a line-scan imaging device 14. The line-scan imaging device 14 includes cameras and a lens system and is positioned above a solar wafer 16 perpendicularly to its surface. An infrared light source 18 is placed below the solar wafer 16 such that infrared light penetrates the solar wafer 16 and reaches the line-scan imaging device 14.
To inspect a 210 mm by 210 mm solar wafer, a 12K line-scan camera is required to have an image resolution better than 210 mm/12,000 pixels or 18 μm/pixel. Based on sampling theorem, this image resolution is only useful for detecting micro-cracks having a crack line width of more than 2 pixels. This means that conventional high speed imaging systems are limited to detecting micro-cracks that has a crack line width larger than 2 pixels×18 μm/pixel or 36 μm. This is a major limitation to conventional high speed imaging systems because the width of micro-cracks is typically smaller than 36 μm.
FIG. 1b shows a close-up view of a micro-crack 20 along a cross-section of the solar wafer 16 at point A of FIG. 1a. The micro-crack 20 has a width smaller than the image resolution 22 of the conventional high speed imaging system 10. As a result, output images of the micro-crack 20 do not have sufficient contrast to allow image analysis software to detect the micro-crack 20.
Other than image resolution problem, detecting micro-crack in solar wafers becomes more complicated when the solar wafer is of multi-crystalline type. Solar wafers are typically fabricated from mono-crystalline or multi-crystalline wafers. Mono-crystalline solar wafers are typically made by cutting single-crystal silicon into slices. Multi-crystalline solar wafers, on the other hand, are obtained by melting a pot of silicon and then allowed the melted silicon to cool slowly before cutting the solidified silicon into slices. Although multi-crystalline solar wafers are lower in quality than mono-crystalline solar wafers due to higher impurity level in the silicon, multi-crystalline solar wafers are nonetheless more cost effective and are becoming more widely used than mono-crystalline solar wafer for making solar cells. Mono-crystalline solar wafers appear to have a uniform surface texture. As shown in FIG. 2, multi-crystalline solar wafers exhibit complicated random surface texture due to the formation of crystal grains of varied size during the solidification process.
The random surface texture in multi-crystalline solar wafers also appears in the output images of the conventional high speed imaging systems 10. Crystal grain boundaries and the contrast between different crystal grains increase the difficulty in detecting the micro-cracks.
There is therefore a need for an improved system and method for facilitating detection of micro-cracks in wafers.